1. Field of the Invention
The present invention relates generally to an improvement relating to a steering gear mechanism, and more particularly to an improved steering gear mechanism principally for use with an automotive vehicle.
2. Description of the Prior Art
Referring to the typical construction of a conventional steering gear system as shown in FIGS. 7 and 8, this system generally comprises a pinion shaft a connected operatively to a steering wheel of an automotive vehicle by way of a steering joint not shown, an oil seal at b, a top cover at c, a lock nut at d, a ball bearing at e and a needle bearing at f which are disposed to rotatably support the pinion shaft in the interior of a rack housing g, respectively, a rack bar at h having a rack portion h.sub.1 engaged with a pinion a.sub.1 provided on the pinion shaft a, a rack support at i provided in the rear side of the rack bar h, a rack support cover at l, a lock nut at k adapted to fix the rack support cover l in position in the rack housing g, a coil spring at j mounted between the rack support cover l and the rack support i, and tie rods at m, m connected operatively with and extending outwardly from the rack bar h in the both right and left directions. The steering gear mechanism with this typical construction is designed to operate in such a manner that the pinion shaft may firstly be caused to be rotated about its axis by way of the steering wheel of an automobile, this rotation motion being converted to a linear motion in the direction Z through the operative engagement between the pinion a.sub.1 on the pinion shaft a and the rack bar h, this motion of the rack bar h being then relayed through the right and left tie rods m, m to corresponding steering wheels of the vehicle, thus having these steering wheels shifted in steering motion to a desired direction of the vehicle to be turned, accordingly.
According to such a typical construction of the steering mechanism, it is known that there would generally occur reaction forces from the physical engagement between the rack h.sub.1 and the pinion a.sub.1 not only in the direction Z of the tie-rod extensions but also in the direction X and Y of the axis of pinion shaft and of the axis extending in the orthognal direction thereto, thus making the efficiency of transmission decreased substantially under the effect of an engagement between the gears' teeth, an elastic deformation in members involved, frictional losses, and the like, among which friction produced as a result of sliding motion between of the rack bar h and the rack support i rendered by a component of force in the direction Y noted above is one of decisive factors in terms of the reduction in transmission efficiency.
Now, reviewing more specifically the effect of this sliding friction existing in the mutual engagement between the rack bar h and the rack support i, assuming a coefficient of friction as .mu..sub.89 and a component of force in the direction Y as FY, a friction force F.sub.89 may be given by the following equation; that is, EQU F.sub.89 =.mu..sub.89 .times.FY
When the rack bar h and the rack support i are placed in an operative engagement relationship with each other as noted above, there is a relationship between a steering force from the steering wheel of a vehicle and a steering angle as shown by an arrow A in FIG. 5. Consequently, a counter sliding frictional force existing between the rack bar h and the rack support i (a steering reaction force existing in the entire steering system) is shown by an arrow A in FIG. 6.
On the other hand, referring to the general requirement in the design of a steering gear system, it is known that there are three principal factors of performance; that is, (1) a requirement of only a small force for a steering operation while the vehicle is standing still and for a large angle steering operation; (2) an exhibition of a proper damping of forces such as friction in or around the neutral position of steering and the like for reducing self-excited vibration (the so-called shimmy vibration) in the steering system; and (3) responsiveness having a feeling of reluctance in or near the neutral position rather than a too much ease in responsiveness during high-speed steering, thus requiring an appropriate extent of rigidity in or near the neutral position of steering of a vehicle.
It is known, however, that the steering system of the conventional construction as shown in FIGS. 5 and 6 would generally suffer from the following drawbacks as apparent from the performance represented in FIGS. 7 and 8. They are: (1) the occurrence of the so-called shimmy condition in the steering system (with the damping forces to damp the vibrations becoming reduced accordingly by a reduction in the coefficient of friction .mu..sub.89 reviewed) for having steering power decreased during a steering operation while standing or for a relatively large angle of steering, together with a elimination of the reluctant feeling in a high-speed steering operation, thus resulting in a reduction in the proper responsiveness to steering; and (2) on the contrary, an increase in the steering power required in the steering operation while standing and for a relatively large angle of steering by an increase in the coefficient of friction .mu..sub.89 or an urging force of the spring j for having the shimmmy conditions decreased and for having an improved property of responsiblity to a high-speed steering operation.
As reviewed fully hereinbefore, when the steering power required is designed to be reduced by decreasing the coefficient of friction, there may inevitably occur the problem of shimmy conditions, and when increasing the coefficient the result is in an increase in the steering power required. In order to cope with such a vicious circle in the design of the steering system, according to the common construction of the steering gear mechanism, it has been the practice that there is employed a certain compromise in the frictional force at the expense of reducing the steering power required.
In consideration of such drawbacks particular to the conventional steering gear mechanism as noted above, it has long been a desire to attain an efficient resolution for overcoming such inevitable problems particular to the conventional steering gear construction.
The present invention is essentially directed to the provision of a due and proper resolution to such inconveniences and difficulties in practice as outlined above and experienced in the conventional steering gear mechanism to by have been left unattended with any proper countermeasures therefor.